The FAR - floor area ratio - of the neighborhood zoning will determine - at least initially - what can be rebuilt on the lot. Check with the City of Fremont planning department for specifics. Normally there are restrictions in an exclusively 1 story neighborhood to add a second story - from height, site lines, shadow lines, privacy issues to the complete dis-allowing of a 2nd story. If you neighborhood has two story homes, you may face an FAR issues anyway if your home is already meeting those maximums. Generally, the larger the lot size, the far easier additions are. Perhaps consider the more traditional 5,000 square foot+ lot sizes of the older homes in the area.